Siccar Point

Kinematic of Siccar Point - A new perspective

 

The Siccar Point unconformity in Scotland is one of the most famous sites in the history of modern geology. Now, the rocks might reveal an unusual phenomenon that happens during the Devonian time. It seems that the Silurian vertical layers were tectonically lifting and shearing up during the Devonian sedimentation.

 

  1. The Silurian strata were punching and deforming the soft Devonian layers above.

  2. The debris flows of the Devonian were chipping off the ancient Silurian bedrock.

  3. Silurian rocks were still under the stress that pervades schistosity.

  4. The liquified Devonian sand was injected between the fragmented Silurian bedrocks.

  5. This injection was hydraulically lifting these fragments.

 

Here a short animation of this new perspective:

 

Here a short animation of the conventional view:

 

The process can be defined as syn-sedimentation tectonics or as syn-tectonics sedimentation. Such an event is unlikely to occur in our present earth with the same magnitude. There are at least 2 consensuses among geologists:

•    The nature of the Devonian deposit tells us that a violent debris flow was raging over the rocky landscape.

•    Many geologists were already stunned by the lack of rock alteration along the unconformity.

 

In geology, we always search for modern examples to explain the phenomenon of the past. If the event was catastrophic, present time might not provide (wishfully!) the right analogy. Unfortunately, we tend to impose our familiar present on an odd past. Around the 1780s, James Hutton was committed to explaining the earth using the benefit offered by the only present we have. That’s how uniformitarianism and actualism came to be the driving paradigm of modern geology. We certainly admire Hutton's innovative mind. With his uniformitarian approach, he couldn't be aware of Siccar Point's singularity. Following the same steps, future generations wouldn't be aware of the complex kinematic of this unconformity.

 

So, to explain the singularity of Siccar Point we must look at modern tectonic activity, named neotectonics. The North Anatolian Fault in Turkey is one of these places on earth where the neotectonics is the most dynamic. Curiously, most neotectonics movements on earth are very different from the ones of the past. The North Anatolian Fault, like most of the active faults on earth, is a strike-slipe fault:  A bloc or plate move laterally relative to another along a vertical boundary.

In our modern time, we do not observe overthrust faults in motion, nor major folding neither nappe (thrust sheet). Erosion defuse efficiently all mountain building agents. What about subduction? Even the modern sediments laid in these trenches are remarkably undisturbed. Although the many devastating earthquakes on earth today, it is naïve to explain the past using actualism.

 

Following the Izmit earthquake (August 17, 1999), I work there as a geophysicist. Some colleagues were digging trenches to study the paleoseismology of this fault line:

 

  • Measuring the fault offset occurring during an ancient earthquake

  • Try to date the paleo seism

  • Looking for the sedimentary phenomenon; liquefaction, landslide, seismite

  • Tectonics erosion

  • Clastic dikes

  • Sand boils or sand volcanoes

Slideshow of some of the phenomena mentioned above; photos from the North Anatolian fault or from general interest:

Meanwhile, these neotectonics elements are still “discreet” compare to modern landslides, floods, tsunamis, and hurricanes.​

Siccar Point and the rest of Scotland was deeply affected by the Caledonian orogeny which encompasses events that occurred from the Ordovician to Early Devonian.

The following map shows the area affected by the Caledonian orogeny. Notice how Europe and North America were reunified before the opening of the Atlantic.

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